1. Field of the Invention
The present invention relates to a technique for depositing a platinum thin film or layer on a substrate as a bottom electrode of a ferroelectric oxide thin film device or a semiconductor device. More particularly, the present invention relates to a technique for controlling the platinum thin film structure to have a preferred (200) orientation in order to improve the electrical properties of an oxide thin film device employing platinum thin films as a bottom electrode for an electronic device.
2. Description of the Prior Art
Semi-conductive, dielectric, ferroelectric, super-conductive and magnetic ceramic materials, used in electronic devices tend to become ever thinner in line with the trends of miniaturization, high-density integration and functional elevation of electronic ceramic parts or devices. The substrates used for electronic ceramic parts can be classified into three types. The first type comprises a single crystal silicon generally called a silicon wafer. The second type comprises the other single crystal materials such as MgO, SrTiO.sub.3 or Al.sub.2 O.sub.3. The third type comprises polycrystal materials such an alumina or diamond.
Poly-silicon has been widely used as a bottom electrode material in tie conventional memory cells without any critical problems. However, it is generally accepted that it can not be used any more as a bottom electrode to manufacture DRAM with over 1G bit and FRAM (Ferroelectric Random Access Memory) which is a new type non-volatile memory. Because high dielectric or ferroelectric oxide thin films such as perovskite structure oxides, bismuth-layered perovskite structure oxides, tungsten-bronze type structure oxide, ReMnO.sub.3 (Re: rare-earth element) and BaMF.sub.6 (M: Mn, Co, Ni, Mg, Zn), should be formed on FRAM, on DRAM devices requiring high degree of integration over 1 G bit, or around the core part of all types of oxide thin film sensors or actuators. However, such high-dielectric oxide films need to be formed under an oxidation atmosphere and high temperature (higher than 500.degree. C.), which may cause problems relating to the polysilicon. For example, if polysilicon is employed as the bottom electrode in a DRAM cell using high-dielectric materials as a capacitor, serious problems may occur due to oxidation of the polysilicon under the high temperature (over 500.degree. C.) and oxidation atmosphere during formation of the high-dielectric oxide thin films. For this reason, platinum is being investigated for substituting for a polysilicon electrode of a memory cell employing a high-dielectric or ferroelectric oxide, because platinum is stable under high temperature and an oxidation atmosphere.
Platinum, known to melt under high temperature of about 1774.degree. C., is thermally stable, not reacting with any other chemical substances. Platinum is expected to overcome the chronic problems (e.g., generation of the residual stress, isolation of the substrate, or variation of the interfacial properties, etc.), which are caused by the lattice mismatch occurring in the formation of thin films due to similarity of the lattice parameter of the platinum to the ferroelectric oxide material of Perovskite structure. Generally, the platinum lattice parameter is 3.9231 .ANG., while the ferroelectric oxide material of Perovskite structure in about 3.9 .ANG..
However, depositing platinum thin films with a conventional method is known to pose a number of problems.
First, it is very difficult to form chemical bonding between the insulating oxide layer and the platinum layer and thug the adhesive strength between the platinum layer and the substrate is weak. One of the attempts that has been made to solve this problem is using a glue layer between the platinum layer and the insulating oxide layer. According to this attempt, a thin film layer composed of one or two elements selected from the group consisting of Ta, Ti, TiN and W in formed on the insulating oxide layer before depositing a platinum thin film layer so that the thin film layer formed on the oxide layer serves as a glue layer between the insulating oxide layer and the platinum layer.
However, employment of this method is known not only to complicate the process of forming a bottom electrode but also to generate additional problems. In the post-process such as annealing treatment or deposition of a ferroelectric oxide layer, the substance of the glue layer diffuses through the platinum layer and reacts with the oxygen of the ferroelectric oxide layer. This disables the platinum thin film from functioning as an electrode and deteriorates the platinum surface smoothness.
According to the prior art, there is also a problem in that hillocks or voids are formed on the platinum layer in the process after the annealing treatment or deposition of the ferroelectric oxide layer. These hillocks or voids cause either shortening of the circuits or heterogeneity in the formation of a high dielectric or ferroelectric oxide layer.
An approach for solving these problems of prior art is disclosed in U.S. Pat. No. 5,736,422 to Lee et al. The invention disclosed in the '422 patent introduced a concept of "depositing platinum in two steps, under an oxygen containing gaseous atmosphere and then under an inert gas atmosphere" into the pertinent technical field. According to one embodiment of the invention, a platinum thin film is deposited on an insulating oxide layer on a silicon wafer in two steps and under different atmospheres. The first step is to deposit an oxygen containing platinum thin film, as opposed to pure platinum, under an oxygen containing atmosphere. The second step is to deposit a pure platinum layer under an inert gas atmosphere. And, the resulting film is annealed. Through this annealing process, the oxygen within the oxygen containing platinum film is removed and the platinum film formed thereby and the additionally deposited platinum film become stable.
According to the description in the '422 patent, "an oxygen containing gaseous atmosphere" is defined as a mixture of inert gas (Ar, Kr, Xe, etc.) with either oxygen or ozone gas mixture. The term, "oxygen containing platinum thin film" is understood to mean a platinum film that oxygen components contained therein partially may form platinum oxide, but most of them, e.g. more than 50% exist within grain boundaries or lattices in the platinum layer structure without being chemically bonded to the platinum. In other words, oxygen is contained in the oxygen containing platinum layer that is not present as platinum oxide.
U.S. Pat. No. 4,511,451 to Sella et. al., discloses a method for forming a thin layer of pure platinum oxide. The term, "platinum oxide" means PtO or PtO.sub.2, although PtO.sub.2 is exampled in the '451 patent, and differs from the term, "oxygen containing platinum" in the strict sense. These terms and definitions are also applicable to the present invention.
It is well known that oxygen may be contained in the depositing atmosphere when employing an insulating thin film on the substrate. However, the oxygen contained in the depositing atmosphere has been regarded as an impure substance harmful for the formation of the platinum thin film. This thought has been changed by the above-mentioned inventions.
The '422 patent discloses a new technique for platinum thin-film deposition process comprising steps of: providing a silicon wafer; forming an insulating oxide layer on a surface of the silicon water; depositing a platinum layer on the insulating oxide layer under an oxidation atmosphere to form an "oxygen containing platinum thin-film"; depositing an additional platinum thin-film to a desired thickness on the oxygen containing platinum thin-film under a complete inert atmosphere; and annealing the silicon substrate at a temperature of 400.degree. C. to 1,300.degree. C. in order to remove oxygen present within the oxygen containing platinum thin-film and to stabilized the entire platinum thin-film.
According to the '422 patent, oxygen and argons gases are introduced into a vacuum chamber to form an oxidation atmosphere and RF power in supplied to the platinum target, thereby an oxygen containing platinum rather than pure platinum thin-film being deposited on the wafer due to oxygen contained within the atmosphere and admixed into the first deposited platinum layer. As well known in the art, during the sputtering process, a part of atmospheric gases are ionized and impinged to the platinum target. As a result, platinum atoms are run out and deposited together with oxygen within the oxidation atmosphere on an object being processed, so that an "oxygen containing platinum thin-film" can be formed on the object.
In addition to the aforementioned disadvantages in the prior art, there still remains another unresolved problem of controlling the structural orientation of the platinum thin film. It is known under the conventional method that the platinum thin film, which is deposited on the oxide insulating layer, generally are not (200)-oriented. As a rule, anisotropic materials have different physical properties depending on their structural orientations.
If the platinum thin film is employed as a bottom electrode in an electronic device, the preferred orientation of the platinum thin film affects the preferred orientation of the material formed on the platinum thin film, ultimately affecting the physical properties of the final device. For instance, if a platinum bottom electrode in (111)-oriented in a process of manufacturing a device utilizing ferroelectric oxide material, the ferroelectric oxide thin film formed on the platinum bottom electrode cannot be oriented to the c-axis direction, thereby deteriorating the working efficiency of the electronic device having the platinum thin film structure as described above and generating the fatigue effect of the device in operation.
It is expected in the pertinent technical field that, if the platinum thin film employed as a bottom electrode is (200)-oriented, the ferroelectric oxide thin film, which in formed on the platinum thin film, will be mostly oriented to the c-axis direction. On the account of this preferred orientation, the working efficiency of the electronic device is expected to be highly increased, while its fatigue effect in expected to be drastically lowered.
For this reason, a great deal of research has been conducted for obtaining a (200)-oriented platinum electrode. It is known, however, that the same result can be achieved by employing a single crystal substrate such as MgO (100) which has a similar lattice parameter to the platinum. However, it is considered impracticable to employ a single crystal substrate such as MgO in manufacturing semiconductor devices such as DRAM or FRAM. This is because processing the substrate itself is not only difficult but also unsuitable to the presently available processes of manufacturing a silicon integrated circuit.
Because of above-mentioned reasons, attempts have been made to employ a conducting oxide electrode such as IrO.sub.2, RuO.sub.2, LSCO, or YBCO as a bottom electrode. In this case, however, problems such as roughening surface or increasing leakage current may also take place. Therefore, no satisfactory research result regarding a (200)-oriented platinum thin film has yet been reported in this field to date.